Condensing heat recovery involves the removal of a significant quantity of heat from waste exhaust gases, to the point where the exhaust gas actually condenses and water vapor drops out as condensate. Because of recent market forces urging energy efficiency, the application of condensing heat recovery has become more popular. Condensing heat recovery systems, or “economizers,” are available as original equipment or for retrofit in a flue gas stack to allow heat energy to be recovered and used. In cylindrical economizers of the prior art, the rising flue gas enters the bottom of the economizer and is channeled upwardly across a tube bundle of a heat exchanger such that liquid condensate forms on the tubes of the heat exchanger and drops downward against the flow of the flue gas. Consequently, a portion of the condensate is re-evaporated by the hot flue gas and does not aid in heat transfer efficiency. Also, in existing designs where the exchanger tubes are in line with the exhaust gas traveling upward and the water being heated is draining downward, it is very difficult to keep the condensate from draining down into the boiler or heat source. Because the condensate is corrosive, its effect on heat source equipment is of great concern.
U.S. Pat. No. 8,006,651 addresses these concerns by providing a “reverse flow” economizer wherein the upwardly flowing hot flue gas is redirected in a downward direction through heat exchange tubes, and then the cooled flue gas is reintroduced into the exhaust path for discharge. A heat exchange medium is introduced through a lower inlet and counter-flows upward through the heat exchange tubes to an upper outlet. Condensate formed on the heat exchange apparatus drops by gravity and is not exposed to hot flue gas, thereby avoiding unwanted re-vaporization of the condensate. The described economizer includes cylindrical inner and outer shells arranged concentrically about a cylindrical flow duct arranged to receive the upwardly flowing flue gas. The inner shell defines a secondary flow passage in which the heat exchange tubes is arranged for cooling the downwardly flowing flue gas, and the outer shell defines a tertiary flow passage by which the cooled flue gas travels upwardly and returns to the primary flow duct. The economizer taught in U.S. Pat. No. 8,006,651 provides an efficient system, but has practical drawbacks. The cylindrical design is laborious to assemble, and the heat exchange tubes reside in an inaccessible zone between the primary flow duct and the outer shell, making maintenance very difficult.
What is needed is an economizer that provides similar operating efficiencies, yet is easier to fabricate and maintain.